You all probably know how some of the D2 vanilla sprites for several monsters including some D1 sprites were used as well.Concerning some of the enemies via anywhere, what are some of the other skill sprites and summons used for monsters. Like I know that the golems of the necromancer were used for monsters like Necrobots, Bartuc's Blood Golems, and Primal Clay, but what about druid oak spirit summons and deadly vines and the sorceress hydra.

This is just something I've always wondered ever since playing median xl fully.

He's asking what original sprites were used in the making of creatures of Median XLU as we know them today. Was, say, a Ni-Va a sprite modified from the old Oak Sage? Just an example. I'm not sure whether they reused sprites or are able to create more without hardcode.

Unlikely. But the origin of new sprites? They are taken from other games. I know for sure some come form IceWind Dale, like the Ice Shards in Baal's room, the Dark dragon from Bremmtown, Clawstorm Terror, the (now new) Dragon Paladin morph and the SuperBeast morph, the giants, the trolls and others too.

well he didn't ask where the new ones come from. he's wondering if monsters like oak/hydra are being used. Answer is no. Why? no reason in particular, although Vine is kind of hard to implement, hydra has no walking mode and Oak is meh.

The vine attack animation could be used for some ranged monster. Like a variant on the lurker type elites that shoot tentacles into the ground that pop up and stab you, except with the final part replaced by an aoe attack that uses the vine animation. Hydra would make for a neat-looking turret.